Specifically, the present invention relates to a biaxially oriented film suitable for, for example, a base film for a magnetic recording medium, particularly a data storage magnetic tape and digital video tape, and suitable for various industrial material films for a capacitor, a heat transfer ribbon, base paper for a thermosensitive stencil printing plate, etc.
Although a large-area film can be continuously produced from polyester, such a film cannot be obtained from other materials. By making use of the properties of polyester films, such as strength, durability, transparency, flexibility, and the property that surface properties can be imparted, polyester films are used in various fields in which the polyester films are greatly demanded, such as the fields of various industrial materials for a magnetic recording medium, a capacitor, a heat transfer ribbon, thermosensitive stencil printing plate base paper, and the like, agricultural materials, packing materials, building materials, etc.
Particularly, from the viewpoint of ease of surface design, the polyester films are used in various fields, and are particularly useful as base films for magnetic recording media. A polyester film is frequently used in the form of a biaxially oriented film in order to improve mechanical properties, thermal properties, electric properties, etc. In a magnetic recording medium, reduction in size of a recording signal has recently been required for decreasing the weight and size of an apparatus and lengthening the recording time. In order to reduce the size of a recording signal and increase the recording density, the surface of a film, particularly the film surface on the magnetic side, is required to have flatness. Furthermore, in a magnetic recording drive, quality is improved by using a MR head, and thus coarse protrusions on the surface of a base film possibly damage the MR head, thereby making severe the requirement for surface flatness of the base film.
On the other hand, with a film having an excessively flat-surface, not only the handling property of the film deteriorates, but also the frictional coefficient between the film surface and a magnetic head is increased in use for a magnetic tape, thereby causing the problems of easily deteriorating the traveling performance of the tape, the durability of the tape surface, and causing wear of the magnetic head.
Therefore, a laminated polyester film (for example, Japanese Unexamined Patent Publication No. 7-272249) containing particles for forming surface protrusions, and coating of a discontinuous film containing fine particles (for example, Japanese Unexamined Patent Publication No. 3208639), etc. are conventionally known.
However, in use of a high concentration of fine particles, the surface energy significantly increases as the particle diameter decreases, and thus the surfaces of the particles must be treated by, for example, coating with a water-soluble polymer in order to suppress aggregation. However, there are the problems of deteriorating the coated film, reducing productivity, and increasing the coat. Even by using these methods, coarse protrusions are inevitably formed by aggregation as the concentration of the particles is increased to some extent, thereby causing difficulties in forming uniform micro protrusions at a high density by using the particles.
Therefore, as a method of forming uniform micro protrusions on the surface without using particles, a method using micro crystals in a laminated portion is known (for example, Japanese Unexamined Patent Publication No. 7-1696). However, this method comprises stretching an unstretched film having a surface layer crystallized in a pre-heating step, and thus has the problem of causing damage due to the drawing rolls used. Therefore, at present, the problems remain unsolved in application to a high-density magnetic recording tape. Other known methods include a method using a polymer for forming a phase separation structure with a non-mesogenic polyester (for example, Japanese Unexamined Patent Publication No. 10-298313). However, in this case, the large size of each of the polymer domains subjected to phase separation is reflected in formation of a rough surface, and under the present condition, this method is thus far short of application to a high-density recording magnetic tape having an evaporated magnetic layer which is required to have a flat surface.